A thermosensitive recording medium develops color by reacting a colorless or pale colored basic leuco dye (henceforth referred to as “dye”) and an electron accepting color developing agent (henceforth referred to as “color developing agent”) when heated and is widely used. In general, a thermal printer equipped with a thermal head is used to record on the thermosensitive recording medium. This recording method has many advantages, such as maintenance free, inexpensive, compact in size, clear color development, etc., therefore is used extensively in facsimiles, printers of computers, automatic ticket vending machines, measurement recorders, handy terminals and the like.
The thermosensitive recording medium used for handy terminals and the like that are mainly used in outdoors is required to be water resistant. The thermosensitive recording medium is also required to have a good color development sensitivity when recorded or forming an image even in a low applied energy by using an energy-saving printer, a high-speed printer and the like.
On the other hand, it is commonly conducted to install a protecting layer (overcoat layer) containing a binder, such as polyvinyl alcohol, on a thermosensitive recording layer in order to improve water resistance of a thermosensitive recording medium. It is well known that a thermosensitive recording medium shows an excellent water resistance and printing run-ability when installing a protecting layer (overcoat layer) comprising a resin containing a carboxyl group, an epichlorohydrin resin and a polyamine/amide resin (Reference 1).
However, when a protecting layer is installed on a thermosensitive recording layer, the heat of a thermal head can not well conducted to the thermosensitive recording layer, which results in a poor color development sensitivity especially when recording in a low applied energy.
In order to address this problem, it is often conducted to make the thermosensitive recording layer contain a carboxyl modified polyvinyl alcohol (References 2, 3) or a combination of a carboxyl modified polyvinyl alcohol and a glyoxal (Reference 4) as a binder without installing a protecting layer.
However, it has been considered problematic to use a resin containing a carboxyl group and a glyoxal, since a resin containing a carboxyl group has a disadvantage of desensitization and a glyoxal is classified as PRTR (Pollutant Release and Transfer Register) Type 1 material which is harmful to the environment.
Reference 1:International Publication W02006/075467Reference 2:Japanese Patent No. 3501308Reference 3:Japanese Patent Application Public Disclosure H06-155916Reference 4:Japanese Patent Application Public Disclosure H06-270547